Need advice on encryption software

I'm currently running Vista Home (oh how I'm tempted to "upgrade" to XP, but that's another story..) and am after a good program to protect a particular folder. Now, I also want to be able to view the various files (around 2000 typically) with the ACDSEE file viewer, so ACDSEE must be ale to display all media files without a problem. Of critical importance as well is that no files be left "in the clear" in the event of a power outage, crash or such.
On the fly encryption/decryption seems ideal as you also don't have to endure the long delay and security risk of the all-at-once encryption and decryption phases which could leave data exposed.

So... can anyone suggest a suitable package to do this ?